Steroid receptors have been shown to serve as substrates in vitro for a cyclic-AMP-dependent protein kinase, and for a calcium-stimulated cytoplasmic kinase. We have recently shown that the avian progesterone receptor is phosphorylated in vivo. More recently, I have found that progesterone receptor can be phosphorylated in vitro by a nuclear kinase which copurifies with the nuclear type II casein kinase. This phosphorylation is greatly enhanced by pretreating the receptor with phosphatase. I propose to follow up this observation as follows: 1) By more thoroughly purifying the kinase and by using inhibitors I will definitely establish whether progesterone receptor is phosphorylated by a nuclear type II casein kinase. 2) I will characterize the in vitro receptor phosphorylation by this kinase. The stoichiometry of phosphate incorporation befores and after phosphatase treatments and the identity of the phosphoamino acids will be investigated. 3) Peptide mapping will be used to estimate the number of phosphorylation sites and to compare the sites phosphorylated in vitro with those phosphorylated in vivo. 4) The effects of phosphorylation and dephosphorylation on the ability of receptor to bind to DNA and to nucleotides will be examined. 5) The ablity of progesterone receptor to serve as a substrate for other protein kinases will be examined. 6) Nuclear phosphatases capable of acting on receptor will be sought. These studies should further our understanding of the ways in which phosphorylation and dephosphorylation affect steroid receptors in the nucleus.